This application is based on, and claims priority to, Japanese application 10-233070, filed Aug. 19, 1998, in Japan, and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an apparatus and method using four wave mixing (FWM) for wavelength conversion.
2. Description of the Related Art
Optical communication systems using fiber optical transmission lines are being used to transmit relatively large amounts of information. However, as users require larger amounts of information to be rapidly transmitted, and as more users are connected to the systems, a further increase in the transmission capacity of optical communication systems is required.
Therefore, wavelength division multiplexing (WDM) is becoming an indispensable technique for increasing the transmission capacity of optical communication systems. In a system adopting WDM, a plurality of optical carriers (channels) having different wavelengths are individually modulated to thereby obtain a plurality of optical signals. These optical signals are then wavelength division multiplexed by an optical multiplexer to obtain WDM signal light. The WDM signal light is then transmitted through a single optical fiber functioning as an optical fiber transmission line. On the receiving side, the WDM signal light is received from the transmission line and then separated into individual optical signals by an optical demultiplexer. Each optical signal is then demodulated to reproduce the data conveyed by the optical signal. Accordingly, by applying WDM, the transmission capacity of a single optical fiber can be increased in accordance with the number of carriers (channels) multiplexed together and transmitted through the fiber. WDM can be contrasted to a conventional optical communication system where a single optical signal is transmitted through an optical fiber.
With an optical communication system employing WDM, it is often necessary to provide wavelength management and wavelength routing control between nodes of the system. For example, as indicated above, a WDM signal includes many wavelengths multiplexed together. In many cases, it may be necessary to convert one wavelength into a different wavelength, or to drop out, or add, wavelengths. Preferably, such wavelength management and wavelength routing control is performed efficiently and at minimum cost.
With an optical communication system employing WDM, wavelength management and wavelength routing control can be realized by performing wavelength identification at each node.
In this respect, a lightwave network which combines WDM and wavelength routing will likely be a significant part of next generation trunk systems. Wavelength conversion will be a key technology in realizing such a lightwave network.
Conventionally known is a method for wavelength conversion utilizing four-wave mixing (FWM). FWM is a third-order nonlinear phenomenon of light. When light enters an optical medium, the light propagates in accordance with the correlation between incident electric field E and polarization density P induced in the optical medium by the incident electric field E. In general, the relation between electric field E and polarization density P is expressed as follows:
xe2x80x83P=xcex50"khgr"E+2dE2+4"khgr"(3)E3+ . . .
The first term is a term indicative of linearity, which shows the fact that the polarization density is proportional to the electric field under the condition where the latter is relatively weak. When the electric field (optical power) becomes strong, the second and subsequent terms are not negligible and most optical media show nonlinear response. The second term is a term indicative of second-order nonlinear effects. This term is negligible in an isotropic medium such as silica (SiO2).
When three optical waves having frequencies of xcfx891, xcfx892, and xcfx893 enter a medium showing third-order nonlinear effects, the following equation holds.
E(xcfx89)=E1exp(ixcfx891t)+E2exp(ixcfx892t)+E3exp(ixcfx893t)
Then, the third-order nonlinear polarization density p=4"khgr"(3)E3 includes a new frequency represented by (xcfx891+xcfx892+xcfx893) and (xcfx891+xcfx892xe2x88x92xcfx893). Letting xcfx894 denote this new frequency, the following equations hold.
xcfx894=xcfx891+xcfx892+xcfx893
xcfx894=xcfx891+xcfx892xe2x88x92xcfx893
Thus, a new optical wave having a frequency xcfx894 is generated. The wavelength conversion is performed on the principle mentioned above.
FIG. 1(A) is a diagram illustrating a conventional technique for wavelength conversion. Referring now to FIG. 1(A), a semiconductor optical amplifier 2 is a nonlinear optical medium having an active layer 4 for guiding optical waves. When signal light having a frequency xcfx89s and pump light having a frequency xcfx89p are input into semiconductor optical amplifier 2, phase-conjugated light having a frequency (2xcfx89pxe2x88x92xcfx89s) is output together with the signal light and the pump light from semiconductor optical amplifier 2. In this manner, wavelength conversion from the signal light to the phase-conjugated light is performed. In the case that the signal light and/or the pump light is/are preliminarily modulated, the phase-conjugated light reflects this modulation.
FIG. 1(B) is a diagram illustrating another conventional technique for wavelength conversion. Referring now to FIG. 1(B), a DFB laser 6 has an active layer 8 for guiding optical waves and a corrugated diffraction grating 10 formed along active layer 8. Diffraction grating 10 may be provided by a refractive index distribution.
In DFB laser 6, pump light is generated by oscillation. Accordingly, phase-conjugated light can be obtained simply by inputting signal light into DFB laser 6.
The techniques for wavelength conversion in FIGS. 1(A) and 1(B) have a problem in that conversion efficiency of the wavelength conversion is low.
Further, high-accuracy positioning with lenses or similar devices is necessary for optical connection of each of semiconductor optical amplifier 2 and DFB laser 6 to optical fibers, so that it is difficult to optically connect each component to the optical fibers with a low connection loss. Particularly, in the technique for wavelength conversion using semiconductor optical amplifier 2 shown in FIG. 1(A), a light source for outputting the pump light is required, thereby complicating the configuration.
Accordingly, it is an object of the present invention to provide an apparatus and method for wavelength conversion with an improved conversion efficiency.
It is another object of the present invention to provide an apparatus and method for wavelength conversion which allows optical components to be easily connected to optical fibers with low connection loss, and has a simple configuration.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing objects of the invention are achieved by providing an optical waveguide doped with a rare earth element and pumped so that an input optical signal is amplified as the input optical signal travels through the optical waveguide. The optical waveguide is provided with at least one light which, together with the input optical signal, causes four wave mixing (FWM) to occur in the optical waveguide. The FWM causes a converted optical signal to be produced in the optical waveguide and having a wavelength different from the input optical signal. The input optical signal is modulated by a transmission signal, so that the converted optical signal is also modulated by the transmission signal. The optical waveguide can be, for example, an erbium doped optical fiber operating as an erbium doped fiber amplifier (EDFA).
Objects of the present invention are also achieved by providing an apparatus which includes a rare earth doped optical fiber, and a pump source. The rare earth doped optical fiber has first and second lights travelling therethrough. The pump source provides pump light to the fiber so that the first light is amplified as the first light travels through the fiber, wherein the first and second lights together cause four wave mixing (FWM) to occur in the fiber. The FWM causes a third light to be produced in the fiber and having a wavelength different from the first light.
Moreover, objects of the present invention are achieved by providing an apparatus which includes a rare earth doped optical fiber having first, second and third lights travelling therethrough. A pump source provides pump light to the fiber so that the first light is amplified as the first light travels through the fiber, wherein the first, second and third lights together cause four wave mixing (FWM) to occur in the fiber. The FWM causes a fourth light to be produced in the fiber and having a wavelength different from the first light.
In addition, objects of the present invention achieved by providing an apparatus which includes a rare earth doped optical fiber, a pump source and a filter. The fiber has first and second lights travelling therethrough. The pump source pumps the fiber so that the first light is amplified as the first light travels through the fiber, wherein the first and second lights together cause four wave mixing (FWM) to occur in the fiber. The FWM causing a third light to be produced in, and output from, the fiber, the third light having a wavelength different from the first and second lights. The filter is optically connected to the fiber to filter light output from the fiber. The filter has a passband which passes the third light and rejects light having a wavelength different from that of the third light. The fiber can be an erbium doped optical fiber. As a result, the apparatus operates as an erbium doped fiber amplifier (EDFA) which amplifies an optical signal, and also uses the principles of FWM to convert the optical signal to a different wavelength for wavelength conversion and wavelength management.
Objects of the present invention are also achieved by providing a method which includes (a) pumping a rare earth doped optical fiber so that a first light travelling through the fiber is amplified; (b) providing a second light which travels through the fiber and, together with the first light, causes four wave mixing (FWM) to occur in the fiber, the FWM causing a third light to be produced in the fiber and having a wavelength different from the first light.
Further, objects of the present invention are achieved by providing an optical communication system which includes a transmitter, a rare earth doped optical fiber, a pump source, a light source and a receiver. The transmitter transmits an optical signal which travels through the fiber. The pump source provides pump light to the fiber so that the optical signal is amplified as the optical signal travels through the fiber. The light source provides a light to the fiber so that the light, together with the optical signal, causes four wave mixing (FWM) to occur in the fiber, the FWM causing a converted optical signal to be produced in the fiber and having a wavelength different from the optical signal transmitted by the transmitter. The receiver receives the converted optical signal from the fiber.